put_if_header_absent function

[rajkumardongre]

This procedure adds the header only if it does not already exist in the current header array. It is typically used internally to add default headers.

[milancurcic]

Thanks @rajkumardongre. The problem here is that we're now duplicating code between this and the http_response module where you also have append_header. I suggest that you define all header related functions in the http_header module, and reuse them from there in request, response, and client modules. The header procedures won't be type bound because they work on arrays of header_type, but they would be defined in the appropriate scope and without duplicating code.

[rajkumardongre]

Thanks @milancurcic for the review. I have migrated all header-related procedures (get_header_value, put_if_header_absent, append_header) to the src/http/http_header.f90 file.

[milancurcic]

Thanks @rajkumardongre. I have simplified the headers API. Rather than introducing a new procedure that does two things (check for presence of key and call append_header), I have removed put_if_header_absent and introduced header_has_key function which simply checks for existence of a key. This makes the API more horizontal in the sense that independent functionality is orthogonal and each procedure does only one thing.

I have also introduced a test suite for headers. Going forward, for any new functionality and module, please also add a test suite program. I have also added and edited existing docstrings in http_header.f90 into FORD-style docstrings. This will allow us easy generation of browsable API docs. Going forward, please use the same dosctring style when documenting functions.

I'm still not 100% happy with how we're setting default headers in the new_request function. Currently we're duplicating the default header keys and values. I'll think about how better to approach this but for now it's fine.