Purpose

Performs some minor code clean-up, which should not noticeably (or at all) change functionality

Description

Notable changes:

• Replace most usages of <code> with {@code ...} in Javadoc
• Add proper summary sentence to GsonBuilder.enableComplexMapKeySerialization
• Extend documentation for GsonBuilder.setDateFormat methods
• Fix TypeToken.isAssignableFrom(Type) throwing AssertionError in some cases
  Maybe the method should not throw an exception in the first place, but it might not matter much since it is deprecated already anyway.
• Remove outdated throws NumberFormatException from internal JsonReader.nextQuotedValue; the behavior had been changed by 85ebaf7
• Fix incorrect documentation on JsonScope fields
• Fix unit tests having 'expected' and 'actual' switched
  For that I removed the .getType() call from TypeToken to use <T> Gson.fromJson(..., TypeToken<T>) instead of <T> Gson.fromJson(..., Type) because otherwise the call to assertThat(...) would have been ambiguous.
• Use dedicated Truth methods instead of isTrue() / isFalse()
• Use common helper methods in JsonWriterTest to avoid duplication
• Implement toString() for ReflectionAccessFilter constants

Checklist

• New code follows the Google Java Style Guide
  This is automatically checked by mvn verify, but can also be checked on its own using mvn spotless:check.
  Style violations can be fixed using mvn spotless:apply; this can be done in a separate commit to verify that it did not cause undesired changes.
• If necessary, new public API validates arguments, for example rejects null
• New public API has Javadoc
  • Javadoc uses @since $next-version$
    ($next-version$ is a special placeholder which is automatically replaced during release)
• If necessary, new unit tests have been added
  • Assertions in unit tests use Truth, see existing tests
  • No JUnit 3 features are used (such as extending class TestCase)
  • If this pull request fixes a bug, a new test was added for a situation which failed previously and is now fixed
• mvn clean verify javadoc:jar passes without errors