Make a copy of extra headers before modifying them (jwt-dotnet#226)

* Make a copy of extra headers before modifying them

Extend RSAlgorithmFactory to support all ctors of RS256Algorithm (jwt…

…-dotnet#221)

* Extend RSAlgorithmFactory to support all ctors of RS256Algorithm
* Update JWT.csproj to bump version

Refactoring RS256Algorithm, tests (jwt-dotnet#223)

* Making GetPublicKey and GetPrivateKey consistent in RS256Algorithm
* Renaming Test to Tests

Added method to add several claims (jwt-dotnet#210)

* Added method to add several claims
* Added unit test for AddClaims method
* Bumped the minor version