Update docs and code to use application/x-tar in the build API

At the "Build image from Dockerfile" section in the API docs
the Content-Type header is missing.
In addition, some parts in the code are still setting the
Content-Type header to application/tar while it was changed
to application/x-tar since 16th September 2015.

Signed-off-by: Boaz Shuster <ripcurld.github@gmail.com>

image_build.go

@@ -29,7 +29,7 @@ func (cli *Client) ImageBuild(ctx context.Context, buildContext io.Reader, optio
 		return types.ImageBuildResponse{}, err
 	}
 	headers.Add("X-Registry-Config", base64.URLEncoding.EncodeToString(buf))
-	headers.Set("Content-Type", "application/tar")
+	headers.Set("Content-Type", "application/x-tar")
 
 	serverResp, err := cli.postRaw(ctx, "/build", query, buildContext, headers)
 	if err != nil {

image_build_test.go

@@ -170,8 +170,8 @@ func TestImageBuild(t *testing.T) {
 					return nil, fmt.Errorf("X-Registry-Config header not properly set in the request. Expected '%s', got %s", buildCase.expectedRegistryConfig, registryConfig)
 				}
 				contentType := r.Header.Get("Content-Type")
-				if contentType != "application/tar" {
-					return nil, fmt.Errorf("Content-type header not properly set in the request. Expected 'application/tar', got %s", contentType)
+				if contentType != "application/x-tar" {
+					return nil, fmt.Errorf("Content-type header not properly set in the request. Expected 'application/x-tar', got %s", contentType)
 				}
 
 				// Check query parameters

plugin_create.go

@@ -12,7 +12,7 @@ import (
 // PluginCreate creates a plugin
 func (cli *Client) PluginCreate(ctx context.Context, createContext io.Reader, createOptions types.PluginCreateOptions) error {
 	headers := http.Header(make(map[string][]string))
-	headers.Set("Content-Type", "application/tar")
+	headers.Set("Content-Type", "application/x-tar")
 
 	query := url.Values{}
 	query.Set("name", createOptions.RepoName)