HTTPX: add send method

[lspgn]

Hello,
Thank you for this library.
In some cases documented by HTTPX, when using directly the send method, the bearer token is not passed.
This change adds a method with a similar signature to request and stream with an auth parameter (those two can likely be removed too).

The only thing I have not fully tested is self.ensure_active_token(self.token) as it's in a non async function.

Thank you

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

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[lspgn]

Replacing with the following fixes the issue but not sure about the solution

            loop = asyncio.get_running_loop()
            asyncio.ensure_future(self.ensure_active_token(self.token), loop=loop)

[lepture]

@lspgn thanks for your work. Maybe you need to add a test case for send method.