http/request: Add :set_form_data method

[daurnimator]

This adds a convenience method for turning a request into a urlencoded form data POST request

[coveralls]

Coverage decreased (-0.07%) to 86.267% when pulling ad28aa3 on request.set_form_data into 929ba0c on master.

[daurnimator]

Why does form data deserve it's own function when there are so many other formats to choose from?
e.g. json, xml, etc. Obviously we don't want to depend on json/xml libraries, so we can't add helpers for all common formats.

See also #7

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The main rationale seems to be that people expect there to be a :set_form_data-like function in a HTTP library.

• luasocket's http has a code-path for it
• prosody has a code-path for it
• curl has a commad line option for it

[coveralls]

Coverage increased (+0.02%) to 86.356% when pulling c8b2c21 on request.set_form_data into 929ba0c on master.

[daurnimator]

I think you need to replace %20 with +.

[daurnimator]

If this gets merged we also need decoding: you can't use query_args as it doesn't decode + to space. Cc @leafo

[coveralls]

Coverage increased (+0.01%) to 86.233% when pulling 903e59c on request.set_form_data into 8b7f4ba on master.

[coveralls]

Coverage increased (+0.01%) to 88.048% when pulling 23840c4 on request.set_form_data into 95b6ec8 on master.