fix(storage): read _base_url instead of _client.base_url inside signed url creation

[o-santi]

What kind of change does this PR introduce?

After #1249, the storage client no longer should read the HTTPx client's base_url, but should instead read self._base_url, as to not mutate objects passed in as shared. However, we are still reading the wrong URL inside create_signed_upload_url.

The tests were passing before because they were also reading the same wrong variable, so the mistakes cancelled out.

Fixes #1294

Additional context

This is certainly confusing. I want to get rid of external httpx clients for v3.0 as quickly as possible.

[coveralls]

Pull Request Test Coverage Report for Build 19042484437

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.567%

---

Totals
Change from base Build 19037859464:	0.0%
Covered Lines:	8755
Relevant Lines:	9458

---

💛 - Coveralls