Rename language fields in Stripe's user agent to start with lang

I noticed this as I was looking at a dashboard that @ob-stripe produced
today. Other languages send language and language version as `lang` and
`lang_version` while we were using the fully expanded `language_*` here.
I'm changing it here for consistency with other libraries, but I'm also
going to patch the server because there are so many Go libraries out
there that could potentially be reporting if we supported both styles.

stripe.go

@@ -111,8 +111,8 @@ type Backends struct {
 type stripeClientUserAgent struct {
 	Application     *AppInfo `json:"application"`
 	BindingsVersion string   `json:"bindings_version"`
-	Language        string   `json:"language"`
-	LanguageVersion string   `json:"language_version"`
+	Language        string   `json:"lang"`
+	LanguageVersion string   `json:"lang_version"`
 	Publisher       string   `json:"publisher"`
 	Uname           string   `json:"uname"`
 }

stripe_test.go

@@ -171,11 +171,11 @@ func TestStripeClientUserAgent(t *testing.T) {
 
 	assert.Empty(t, userAgent["application"])
 	assert.Equal(t, "go", userAgent["language"])
-	assert.Equal(t, runtime.Version(), userAgent["language_version"])
+	assert.Equal(t, runtime.Version(), userAgent["lang"])
 
 	// Anywhere these tests are running can reasonable be expected to have a
 	// `uname` to run, so do this basic check.
-	assert.NotEqual(t, stripe.UnknownPlatform, userAgent["language_version"])
+	assert.NotEqual(t, stripe.UnknownPlatform, userAgent["lang"])
 }
 
 func TestStripeClientUserAgentWithAppInfo(t *testing.T) {