Fix some cases of merging with base_url

[tomchristie]

Use a more resilient implementation for merging against a base_url.

Prevents the merge URL from being treated as absolute in some cases.

Eg, previously...

>>> client = httpx.Client(base_url="https://example.org/some/path")
>>> client.build_request("GET", "/sub:path")
<Request('GET', 'sub:path')>

Is now fixed...

>>> client = httpx.Client(base_url="https://example.org/some/path")
>>> client.build_request("GET", "/sub:path")
<Request('GET', 'https://example.org/some/path/sub:path')>

Also now deals more correctly with joining relative URLs, eg...

httpx.URL("/path/to/somewhere/").join("../somewhere-else")
URL('/path/to/somewhere-else')

Closes #1498

[florimondmanca]

Super nice. Worth adding a test case with that special path containing : in #1498?

[tomchristie]

Yup, got one for that... test_merge_relative_url_with_path_including_colon