fix #292 change get_args to return mutliple values per key

[stuart-byma]

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Identify the Bug

https://github.com/etr/libhttpserver/issues/292

Description of the Change

Change arg_map and arg_view_map to std::map<std::string, std::vector<std::string>> etc, so that multiple values per key can be returned.

Update an appropriate test case to check that the behavior is correct.

Alternate Designs

No alternate designs

Possible Drawbacks

Small change to user facing API.

Verification Process

Tests were updated to check that new code is correct.

Release Notes

• Fixed a bug where http_request::get_args() only returned the last value for a given argument key. It now returns all values associated with a given key.