Fix setSignedPreKey request - don't expect a JSON response (#1595)

Don't expect a json response from this endpoint. It will only give you
empty responses and exceptions. And sadness.

// FREEBIE

js/libtextsecure.js

@@ -37745,7 +37745,6 @@ var TextSecureServer = (function() {
             return this.ajax({
                 call                : 'signed',
                 httpType            : 'PUT',
-                responseType        : 'json',
                 jsonData            : {
                     keyId: signedPreKey.keyId,
                     publicKey: btoa(getString(signedPreKey.publicKey)),