HTTP client steps: remove combinatorial complexity

[farbodsaraf]

There can be many combinations of headers, body, etc. when composing a request:

• no headers, no body
• no headers, with body inline (in the .feature file)
• ho headers, with body from file
• with headers, no body
• with headers
• with headers, with inline form parameters body (table in the .feature file)
• with headers, multipart body
• etc.

The current approach is to have a step for each combination. This leads to an explosion in the number of required steps.
Refactor to allow the request to be composed using multiple steps:

Given   the request headers
      | Content-Type | application/x-www-form-urlencoded |
And   the form body parameters
      | grant_type | password            |
      | client_id  | com.farbodsaraf     |
 When  I call 'POST' on '/authserver/oauth/token'

This way, you can compose whatever request is needed, and the code remains maintainable.

The intermediary information (headers, body, etc.) can be saved in story variables.
Important note: the steps that perform the actual request should clear all such story variables, so that another request in the same scenario will not be affected.