Fix cargo doc with custom build commands

There's no "doc-all" profile, so it needs to be canonicalized when finding the
name of the profile to pass to a build command.
auto merge of #580 : alexcrichton/cargo/fix-cargo-doc, r=brson

There's no "doc-all" profile, so it needs to be canonicalized when finding the
name of the profile to pass to a build command.