add `return` statement to all `sendRequest()` calls

The `wpcom-xhr-request` and `wpcom-proxy-request` modules are
in the process of being updated to return XMLHttpRequest instances,
so return them in wpcom.js as well so that the user can leverage
XHR features (i.e. "progress" events).
package: update "wpcom-xhr-request" to v0.3.0
Merge pull request #55 from Automattic/update/add-return-calls

add `return` statement to all `sendRequest()` calls