Enable content__contains pattern

[rjprins]

Thanks for the awesome lib! :pray: I stumbled on some test cases where everything was the same except for the request contents and the contents only differed in some small details. This change seemed simple enough.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (b014780) 100.00% compared to head (082940c) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #236 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 20 20 Lines 2756 2758 +2 Branches 417 417 ========================================= + Hits 2756 2758 +2 ``` | [Impacted Files](https://app.codecov.io/gh/lundberg/respx/pull/236?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jonas+Lundberg) | Coverage Δ | | |---|---|---| | [tests/test\_patterns.py](https://app.codecov.io/gh/lundberg/respx/pull/236?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jonas+Lundberg#diff-dGVzdHMvdGVzdF9wYXR0ZXJucy5weQ==) | `100.00% <ø> (ø)` | | | [respx/patterns.py](https://app.codecov.io/gh/lundberg/respx/pull/236?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jonas+Lundberg#diff-cmVzcHgvcGF0dGVybnMucHk=) | `100.00% <100.00%> (ø)` | |

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[rjprins]

Friendly nudge :ping_pong: I think this is ready to merge.