fix(azidentity): do not strip away request headers in doForClient

Some authorities might require certain headers to be passed.

For example, in our dSTS auth flow, the request form contains
client_info, which needs to be accompanied by X-Client-SKU=MSAL.Go
header, else the API call produces

AADSTS501791: Client_info is only supported for MSAL/ADAL,
please ensure that MSAL/ADAL custom headers are being sent.

The `doForClient` function creates new `runtime.Request` from the incoming
request, but it fails to propagate the respective headers.

This commits is addressing that.

Signed-off-by: HandsomeJack <dusek.honza@gmail.com>

sdk/azidentity/CHANGELOG.md

@@ -8,6 +8,7 @@
 ### Breaking Changes
 
 ### Bugs Fixed
+* `azidentity.doForClient` method no longer removes headers from the incoming request
 
 ### Other Changes
 

sdk/azidentity/azidentity.go

@@ -151,6 +151,17 @@ func doForClient(client *azcore.Client, r *http.Request) (*http.Response, error)
 			return nil, err
 		}
 	}
+
+	// copy headers to the new request, ignoring any for which the new request has a value
+	h := req.Raw().Header
+	for key, vals := range r.Header {
+		if _, has := h[key]; !has {
+			for _, val := range vals {
+				h.Add(key, val)
+			}
+		}
+	}
+
 	resp, err := client.Pipeline().Do(req)
 	if err != nil {
 		return nil, err

sdk/azidentity/azidentity_test.go

@@ -1075,6 +1075,10 @@ func TestDoForClient(t *testing.T) {
 					assert.Empty(t, rb)
 				}
 
+				for k, v := range tt.headers {
+					assert.Equal(t, v, req.Header[k])
+				}
+
 				assert.Equal(t, policyHeaderValue, req.Header.Get(policyHeaderName))
 
 				rw.Header().Set("content-type", "application/json")