Added 2 matchers for headers

[ghost]

Added a matcher to check if an NSURLRequest has a header set for a key
Added a matcher to check if an NSURLRequest has a header set for a key and that it's value matches a value passed in.

[AliSoftware]

Great thanks for the PR! 👌

Could you add an entry (under some ## Master title) in the CHANGELOG.md to credit yourself? Thx!

[ghost]

Awesome!

Sorry not sure I understand what you mean by some ## Master title. Could you possibly give an example?

[AliSoftware]

The CHANGELOG have titles for each version, like ## 4.8.0 and ## 4.7.0 etc.
So tTo add a new entry, you'll simply need to add a section for "Master" (rather than, e.g. add a section named "4.9.0" or whatnot — since I don't know when I'll release a new version and what this next version will contain, so we can't know in advance what the next version number/name is gonna be).

## Master

* <Description of your changes here>.  
  [@<yourGitHubHandle>](<YourGitHubProfileURL>), [#160](https://github.com/AliSoftware/OHHTTPStubs/pull/160)

Note: in order for GitHub's markdown to go to the next line between your entry description and the line with your name and PR link, you'll have to end the description line with two spaces (see other entries in the CHANGELOG for inspiration)

[ghost]

Great, thank you! Will do that now.

[AliSoftware]

👍