fix(rt): Swap User-Agent and x-amz-user-agent headers (#158)

Author: kggilmer

client-runtime/protocols/http/common/src/aws/sdk/kotlin/runtime/http/middleware/UserAgent.kt

@@ -38,8 +38,12 @@ public class UserAgent(private val awsUserAgentMetadata: AwsUserAgentMetadata) :
 
     override fun <I, O> install(operation: SdkHttpOperation<I, O>) {
         operation.execution.mutate.intercept { req, next ->
-            req.subject.headers[USER_AGENT] = awsUserAgentMetadata.userAgent
-            req.subject.headers[X_AMZ_USER_AGENT] = awsUserAgentMetadata.xAmzUserAgent
+            // NOTE: Due to legacy issues with processing the user agent, the original content for
+            // x-amz-user-agent and User-Agent is swapped here.  See top note in the
+            // sdk-user-agent-header SEP and https://github.com/awslabs/smithy-kotlin/issues/373
+            // for further details.
+            req.subject.headers[USER_AGENT] = awsUserAgentMetadata.xAmzUserAgent
+            req.subject.headers[X_AMZ_USER_AGENT] = awsUserAgentMetadata.userAgent
             next.call(req)
         }
     }

client-runtime/protocols/http/common/test/aws/sdk/kotlin/runtime/http/middleware/UserAgentTest.kt

@@ -20,6 +20,7 @@ import software.aws.clientrt.http.sdkHttpClient
 import software.aws.clientrt.time.Instant
 import software.aws.clientrt.util.get
 import kotlin.test.Test
+import kotlin.test.assertEquals
 import kotlin.test.assertTrue
 
 class UserAgentTest {
@@ -53,5 +54,7 @@ class UserAgentTest {
         val request = op.context[HttpOperationContext.HttpCallList].last().request
         assertTrue(request.headers.contains(USER_AGENT))
         assertTrue(request.headers.contains(X_AMZ_USER_AGENT))
+        assertEquals("aws-sdk-kotlin/1.2.3", request.headers[X_AMZ_USER_AGENT])
+        assertTrue(request.headers[USER_AGENT]!!.startsWith("aws-sdk-kotlin/1.2.3 api/test-service/1.2.3"))
     }
 }