Change `blob.compose` to fit API behavior

[cojenco]

Currently blob.compose() is misusing if_generation_match as a list of sourceObjects[].objectPreconditions.ifGenerationMatch sent with the API request body. Based on the doc, if_generation_match is an optional query parameter that makes the operation conditional only when the composed object's current generation matches the given value.

[tritone]

Just to clarify-- you mean that you think if_generation_match should be applied to the destination object and not the source objects, correct? This is a behavior change so it definitely requires some scrutiny (unless there's no way it could've worked as it is currently).

[cojenco]

@tritone Right! According to the API doc, if_generation_match should be applied to the destination object, and serves as the preconditions for retries.