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.
* @since 20.0
   */
  public static final MediaType WMA_AUDIO = createConstant(AUDIO_TYPE, "x-ms-wma");
  /**
   * Windows Media metafiles. For more information, see <a
   * href="https://msdn.microsoft.com/en-us/library/windows/desktop/dd562994(v=vs.85).aspx">file
   * name extensions for Windows Media metafiles</a>.
   *
   * @since 20.0
   */
  public static final MediaType WAX_AUDIO = createConstant(AUDIO_TYPE, "x-ms-wax");
  /**
   * Real Audio. For more information, see <a
   * href="http://service.real.com/help/faq/rp8/configrp8win.html">this link</a>.
   *
   * @since 20.0
   */
  public static final MediaType VND_REAL_AUDIO = createConstant(AUDIO_TYPE, "vnd.rn-realaudio");
  /**
   * WAVE format, as defined by <a href="https://tools.ietf.org/html/rfc2361">RFC 2361</a>.
   *
   * @since 20.0
   */
  public static final MediaType VND_WAVE_AUDIO = createConstant(AUDIO_TYPE, "vnd.wave");
  
  public static final MediaType MP4_VIDEO = createConstant(VIDEO_TYPE, "mp4");
  public static final MediaType MPEG_VIDEO = createConstant(VIDEO_TYPE, "mpeg");
  public static final MediaType OGG_VIDEO = createConstant(VIDEO_TYPE, "ogg");
  public static final MediaType QUICKTIME = createConstant(VIDEO_TYPE, "quicktime");
  public static final MediaType WEBM_VIDEO = createConstant(VIDEO_TYPE, "webm");