Filter test inputs for presigning header exclusion proptest (#2986)

## Motivation and Context
During our internal build, the `aws-sigv4` crate failed on a proptest as
follows:
```
Diff < left / right > :
<content-length;content-type;host
>content-length;content-type

.
minimal failing input: excluded_headers = [
    "host",
]
        successes: 74
        local rejects: 0
        global rejects: 0
', aws-sigv4/src/http_request/canonical_request.rs:838:5

failures:
    http_request::canonical_request::tests::presigning_header_exclusion_with_explicit_exclusion_list_specified
```
This says that `excluded_headers` should not contain `host`.

To address it, this PR will filter out test inputs from proptest that
contain `content-length`, `content-type`, or `host` (all of which appear
in the expected value for the test at line 886 in the new revision).

## Testing
No new tests have been added, relied on the existing tests in CI

----

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