Generate Snippets for Multiple Content-Types

[bpross]

Adds support for generating snippets for endpoint methods that support multiple content-types. Endpoints might support uploading files using multipart/form-data, but otherwise support application/json for other modifications. As it stands this library has the following behavior:

• Generate a snippet for the first content-type defined in path.method.requestBody.content
• Set the content-type header to be the last content-type defined in path.method.requestbody.content

This PR address those concerns by:

• Generating a code snippet for each of the content-types defined in path.method.requestBody.content
• Add support for multipart/form-data with accompanying tests
• Adding mimeType to the snippet object to provide uniqueness of snippets
• Removing the parsing of content-type headers in `getHeaders.
• Setting content-type headers for HARs that have postData
• Simplify getPayloads to loop over supported content-types instead of hard coding blocks. This will ease support for more content-type support in the future

[bpross]

@ErikWittern do you mind taking a look when you have a change? Thanks!

[ErikWittern]

@bpross I just merged #71, which introduced a conflict with this PR. Would you be willing to resolve the conflict?

[bpross]

@ErikWittern conflict resolved.