feat: Pass custom headers to tile server requests

[jthetzel]

This pull request allows contextily users to pass an optional headers parameter that will be included in the _retryer function's request.get call to the tile server.

We use titiler with cloud optimized geotiffs for tiles and often authenticate the titiler server with an authentication bearer token in the http request headers. This change allows us to use contextily with our authenticated titiler.

Opening this pull request for discussion in case it is of use to others. Please let me know if there is a simpler way to use contextily with custom headers.

Related issue: #267

[jthetzel]

This also updates two unrelated pytest expected sums which were failing ( 4cfa350 ).

I can reproduce the pytest failure in the main branch head ( 851b77b ) to confirm it is unrelated to this pull request, to the best of my understanding.

[jthetzel]

This also removes an unrelated matplotlib pytest warning ( ad65bcb ).

[martinfleis]

Thanks! I think this approach is fine. Just one comment - can we catch a situation when a user passes custom user-agent in headers?

[jthetzel]

Thanks, @martinfleis . Which behavior do you prefer?

1. Silently override custom user-agent with contextily user-agent (currently in this PR),
2. Print warning and override custom user-agent with contextily user-agent,
3. Silently use custom user-agent, ignoring contextily user-agent,
4. Print warning and use custom user-agent, ignoring contextily user-agent, or
5. Raise exception.

My very mild preference is for option 3 to pass responsibility to the user, but I defer to you. I implemented option 1, because I was unsusre of the significance of the contextily USER_AGENT.

[martinfleis]

I would go with the option 3. We generate it to essentially get a unique identifier of the process that is fetching the tiles. I am happy to let users override this without a warning.

[jthetzel]

After review, option 3 was already implemented. Added a test to confirm the user provided user-agent overrides the contextily user-agent ( 546fdc8 ).

[martinfleis]

Thanks!