Remove expectation that GET without a body has content-length 0.

[aam]

See https://dart-review.googlesource.com/c/sdk/+/194881 that is going to remove content-length:0 header for GET and HEAD requests that has no body.

[kevmoo]

CC @natebosch

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[natebosch]

@googlebot I consent.

[natebosch]

Not sure why my commit confused the CLA bot...