Python client should support overriding special HTTP headers

Description

The python client should allow overriding the Accept and Content-Type headers by the caller of API functions.

Right now, the Accept header is automatically set when 'produces' is defined in the service description and Content-Type is set when 'consumes' is defined. On the other hand, an end user might want to explicitly specify the return format of a certain request. This is the case with file downloads in a given format.

Swagger-codegen version

 > java.exe -jar .\swagger-codegen-cli.jar version
 2.3.1

Swagger declaration file content or url

Any file that has produces and/or consumes definitions.

Suggest a fix/enhancement

The easiest solution is to set the default Accept and Content-Type before looping through the actual parameters. This way, if the user specifies the accept header, it would simply overwrite the default. Same applies to Content-Type. This would involve moving a few lines starting from https://github.com/swagger-api/swagger-codegen/blob/03ed4334f81dea6329065a214a2429ded92190ff/modules/swagger-codegen/src/main/resources/python/api.mustache#L183 before https://github.com/swagger-api/swagger-codegen/blob/03ed4334f81dea6329065a214a2429ded92190ff/modules/swagger-codegen/src/main/resources/python/api.mustache#L163

I can imagine more sophisticated approaches, however. For example, passing on the entire kwargs or all_params to self.api_client.call_api would allow overriding behavior at a single place (by overriding function in ApiClient.

Start in modules/swagger-codegen/src/main/resources/python/api.mustache, around the linked header setup and parameter loop. Check how Accept and Content-Type defaults are assigned and how caller-supplied headers are processed. Done means generated Python API functions let callers override those defaults, including for file downloads; validate against a generated client from a declaration containing produces or consumes.