I've found cases where `MediaType::toString` generates erroneous results.

One case is when the `MediaType` instance has a parameter of an empty value. This can happen when calling `withParameter("foo", "")`, or parsing `text/plain; foo=""`. When the parameters are later joined, the value is erroneously matched as a token instead of being quoted (as tokens cannot be empty). This will lead the following to throw `IllegalArgumentException`: 
```java
MediaType type1 = MediaType.parse("text/plain; foo=\"\"");
MediaType type2 = MediaType.parse(type1.toString()); // Trying to parse 'text/plain; foo=' will fail
```
 I think [this line](https://github.com/google/guava/blob/5a8f19bd3556012ed9e65cd4268a85ddde95733f/guava/src/com/google/common/net/MediaType.java#L1091) should first check if the string is empty before matching against `TOKEN_MATCHER`, or maybe just return the literal `"\"\""` on that case to avoid the `StringBuilder` allocation.

Another case is when parameters are set explicitly using `withParameter[s](...)`, where parameter value[s] contain non-ASCII characters. For example this will also throw `IllegalArgumentException`:
```java
MediaType type1 = MediaType.create("text", "plain").withParameter("let_me_in!", "\"جوافة\"");
MediaType type2 = MediaType.parse(type1.toString()); // The tokenizer will fail to consume QUOTED_TEXT_MATCHER
```
I don't know whether this is the intended behaviour or not but `MediaType::toString` can be used in HTTP header values which shouldn't contain non-ASCII octets. IMHO `normalizeParameterValue(...)` should validate the value against the `ascii()` matcher.
* UTF-8 encoded <a href="https://en.wikipedia.org/wiki/Wireless_Markup_Language">Wireless Markup
   * Language</a>.
   *
   * @since 13.0
   */
  public static final MediaType WML_UTF_8 = createConstantUtf8(TEXT_TYPE, "vnd.wap.wml");
  /**
   * As described in <a href="http://www.ietf.org/rfc/rfc3023.txt">RFC 3023</a>, this constant
   * ({@code text/xml}) is used for XML documents that are "readable by casual users." {@link
   * #APPLICATION_XML_UTF_8} is provided for documents that are intended for applications.
   */
  public static final MediaType XML_UTF_8 = createConstantUtf8(TEXT_TYPE, "xml");
  /**
   * As described in <a href="https://w3c.github.io/webvtt/#iana-text-vtt">the VTT spec</a>, this is
   * used for Web Video Text Tracks (WebVTT) files, used with the HTML5 track element.
   *
   * @since 20.0
   */
  public static final MediaType VTT_UTF_8 = createConstantUtf8(TEXT_TYPE, "vtt");
  /**
   * <a href="https://en.wikipedia.org/wiki/BMP_file_format">Bitmap file format</a> ({@code bmp}
   * files).
   *
   * @since 13.0
   */
  public static final MediaType BMP = createConstant(IMAGE_TYPE, "bmp");
  /**
   * The <a href="https://en.wikipedia.org/wiki/Camera_Image_File_Format">Canon Image File
   * Format</a> ({@code crw} files), a widely-used "raw image" format for cameras. It is found in
   * {@code /etc/mime.types}, e.g. in <a href=