Allow first path segments containing colons

[hannseman]

Using urljoin for merging path segments where the first segment contains a colon will cause the base to be ignored.

>>> urljoin("/", "foo:bar")
'foo:bar'

See RFC3986

A path segment that contains a colon character (e.g., "this:that") cannot be used as the first segment of a relative-path reference, as it would be mistaken for a scheme name. Such a segment must be preceded by a dot-segment (e.g., "./this:that") to make a relative-path reference.

The easiest way is probably to not use urljoin and simply add a leading / when needed.