Makes the content types that should be considered "text" configurable

[ktraunmueller]

With the current VCRURLConnection master, responses from jsonApi endpoints will be saved base64-encoded to the cassette json. This is a pain when writing tests against the content returned in such responses.

With this pull request, the list of content types that are considered text (and therefore won't get base64-encoded when saved to file) has been made configurable.

In our tests (Swift), we're setting it to

VCR.textContentTypes = ["application/json",  "application/vnd.api+json"]

which will result in human-readable JSON in the cassettes.

[dstnbrkr]

This is great - thank you!