Remove unnecessary for loop in Session.send()

[jacobvosmaer]

This removes a for loop around io.Writer.Write() calls in session.send(). The loop looks like what you would write in C when making write(2) syscalls. With C and write(2), you need to account for partial writes and call write(2) more than once. This is not how Go's io.Writer works. The io.Writer contract clearly states that "Write must return a non-nil error if it returns n < len(p)". Dealing with partial writes is the responsibility of implementations of io.Writer, not that of callers.

Removing the for loop makes the code simpler and more idiomatic.

[hashicorp-cla]

All committers have signed the CLA.

[jefferai]

Thanks!