Prefer localhost over 0.0.0.0 when in "development" mode - #1699

This issue is somewhat tricky. When Rails is booted via `rails server` there are two types of configuration options passed, ones specified directly by a user like `rails s -p 3001` will always "win".

For any other config that is not explicitly passed in, puma will consider it a "default". For example when you run `rails s` (without -p) then the default port will be 3000.

There is one other way to configure puma though, and that is via a config file:

```
# config/puma.rb
port 3002
```

This is the order of precedence for configuration

1) Anything the user explicitly passes to `rails s`
2) Config specified in `config/puma.rb` file
3) Default values passed in via `rails s`
4) Defaults values stored in puma

This fallback mechanism works well except in the case of calling `port` in a `config/puma.rb` file. To understand look at the [old method definition](https://github.com/puma/puma/blob/2668597ec1dd9546d83db9f2ec5ad092add483e6/lib/puma/dsl.rb#L140-L145):

```
def port(port, host=nil)
  host ||= Configuration::DefaultTCPHost
  bind "tcp://#{host}:#{port}"
end
```

When the `port` method gets called, even if the user did not specify a `host` the `Configuration::DefaultTCPHost` will be used, which is a problem for local development because it defaults to `0.0.0.0`. [SO about 0.0.0.0 versus localhost](https://stackoverflow.com/questions/20778771/what-is-the-difference-between-0-0-0-0-127-0-0-1-and-localhost).

In this case, while a user did directly specify a port, they did not specify a host, so you would expect the `rails s` defaults passed in to take affect.

To make Puma respect that the host coming from `rails s` has more precedence than it's own default host, we must introduce the ability to set and retrieve a default_host value.

This is then used in the rack handler so when `rails s` passes in `:Host => "localhost"` then it is used instead of reverting to `0.0.0.0`.

The issue with puma#1699 is the test was wrong, it would have failed if a config file was present with a `port` invocation.

This issue is somewhat tricky. When Rails is booted via `rails server` there are two types of configuration options passed, ones specified directly by a user like `rails s -p 3001` will always "win".

For any other config that is not explicitly passed in, puma will consider it a "default". For example when you run `rails s` (without -p) then the default port will be 3000.

There is one other way to configure puma though, and that is via a config file:

```
# config/puma.rb
port 3002
```

This is the order of precedence for configuration

1) Anything the user explicitly passes to `rails s`
2) Config specified in `config/puma.rb` file
3) Default values passed in via `rails s`
4) Defaults values stored in puma

This fallback mechanism works well except in the case of calling `port` in a `config/puma.rb` file. To understand look at the [old method definition](https://github.com/puma/puma/blob/2668597ec1dd9546d83db9f2ec5ad092add483e6/lib/puma/dsl.rb#L140-L145):

```
def port(port, host=nil)
  host ||= Configuration::DefaultTCPHost
  bind "tcp://#{host}:#{port}"
end
```

When the `port` method gets called, even if the user did not specify a `host` the `Configuration::DefaultTCPHost` will be used, which is a problem for local development because it defaults to `0.0.0.0`. [SO about 0.0.0.0 versus localhost](https://stackoverflow.com/questions/20778771/what-is-the-difference-between-0-0-0-0-127-0-0-1-and-localhost).

In this case, while a user did directly specify a port, they did not specify a host, so you would expect the `rails s` defaults passed in to take affect.

To make Puma respect that the host coming from `rails s` has more precedence than it's own default host, we must introduce the ability to set and retrieve a default_host value.

This is then used in the rack handler so when `rails s` passes in `:Host => "localhost"` then it is used instead of reverting to `0.0.0.0`.

The issue with puma#1699 is the test was wrong, it would have failed if a config file was present with a `port` invocation.