Fix storage content-type on desktop, and add test.

[jonsimantov]

Description

Provide details of the change, and generalize the change in the PR title above.

Storage desktop: When uploading a file with custom metadata, if the metadata specifies a content-type, use that as the content-type for the HTTP request.

This should fix firebase/quickstart-unity#1107, which is failing because of the Content-Type of the HTTP upload.

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Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

Add upload of image file to integration test. (In the future, this could be extended to support image resize, etc.)

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[github-actions]

Integration test with FLAKINESS (succeeded after retry)

Requested by @jonsimantov on commit 0a0c4fc
Last updated: Mon Apr 25 18:34 PDT 2022
View integration test log & download artifacts

Failures	Configs
firestore	[TEST] [FLAKINESS] [Android] [1/3 os: macos] [1/2 android_device: android_target] (1 failed tests)   CRASH/TIMEOUT

Add flaky tests to go/fpl-cpp-flake-tracker

[jonsimantov]

Test failure may not be related to this, but I'll investigate.

[jonsimantov]

Test failure may not be related to this, but I'll investigate.

Was related - caused by multithreaded access. Fixed.

[AlmostMatt]

Looks good, just a couple minor comments/questions.