Fix auto-headers for aiohttp (for real this time :))

[lmazuel]

In some scenario, aiohttp is setting content-type for us. This might be something we don't want, for instance for Storage it was breaking the signature (Content-Type being part of the signature, the signature didn't match anymore). First attempt to fix was then to say to aiohttp "whatever scenario, don't do auto-headers": #6992

But this one broke the formdata tests: because when using "multipart/form-data" you really want aiohttp to do that for you (there is a boundary to generate, etc.).

So this PR is being more explicit: if we know for sure this request comes with no body: don't let aiohttp add automatically "Content-Type". I believe it's more accurate to what we wanted to fix anyway.

Side note @scbedd : if this issue #4773 was implemented, we would have caught that earlier :/. No blaming, just to give context for the value that would bring #4773

[AutorestCI]

(message created by the CI based on PR content)

Installation instruction

Package azure-core

You can install the package azure-core of this PR using the following command:
pip install "git+https://github.com/Azure/azure-sdk-for-python@auto_header_aiohttp_fix#egg=azure-core&subdirectory=azure-core"

You can build a wheel to distribute for test using the following command:
pip wheel --no-deps "git+https://github.com/Azure/azure-sdk-for-python@auto_header_aiohttp_fix#egg=azure-core&subdirectory=azure-core"

If you have a local clone of this repository, you can also do:

• git checkout auto_header_aiohttp_fix
• pip install -e ./azure-core

Or build a wheel file to distribute for testing:

• git checkout auto_header_aiohttp_fix
• pip wheel --no-deps ./azure-core

Direct download

Your files can be directly downloaded here:

• azure-core-1.0.0b4.zip
• azure_core-1.0.0b4-py2.py3-none-any.whl