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>
Upstream-commit: 0247b1509c
Component: cli

components/cli/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)