Add gzip and deflate decoder for json content type

[dszakallas]

This pull request adds corresponding decoders when content encoding is 'gzip'or'deflate'`, but only if the content type is JSON.

Motivation:
currently, nock tries to parse JSON from the requestBody even if it is compressed and ultimately fails with a SyntaxError:

SyntaxError: Unexpected token f in JSON at position 1
    at Object.parse (native)
    at RequestOverrider.end (/omitted/node_modules/nock/lib/request_overrider.js:298:28)

This pull request solves this issue and in the request handler you get a decoded/parsed JSON. Behaviour for other content types should be unaffected.
As it uses sync API, it only works above node >=0.12

[vrinek]

Hello and sorry for the lack of response. Please rebase on top of (or merge in) latest master as the tests are (finally) green again.

[coveralls]

Coverage decreased (-0.05%) to 96.531% when pulling b3d4081 on szdavid92:feat/gzip-content-type-decoder into 192f0f8 on node-nock:master.

[dszakallas]

Done!

[vrinek]

Thanks for the contribution 😃

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue and add a reference to this one if it’s related. Thank you!