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.
* Serialization</a>.
   *
   * @since 27.1
   */
  public static final MediaType JOSE_JSON = createConstant(APPLICATION_TYPE, "jose+json");
  public static final MediaType JSON_UTF_8 = createConstantUtf8(APPLICATION_TYPE, "json");
  /**
   * The <a href="http://www.w3.org/TR/appmanifest/">Manifest for a web application</a>.
   *
   * @since 19.0
   */
  public static final MediaType MANIFEST_JSON_UTF_8 =
      createConstantUtf8(APPLICATION_TYPE, "manifest+json");
  /**
   * <a href="http://www.opengeospatial.org/standards/kml/">OGC KML (Keyhole Markup Language)</a>.
   */
  public static final MediaType KML = createConstant(APPLICATION_TYPE, "vnd.google-earth.kml+xml");
  /**
   * <a href="http://www.opengeospatial.org/standards/kml/">OGC KML (Keyhole Markup Language)</a>,
   * compressed using the ZIP format into KMZ archives.
   */
  public static final MediaType KMZ = createConstant(APPLICATION_TYPE, "vnd.google-earth.kmz");
  /**
   * The <a href="https://tools.ietf.org/html/rfc4155">mbox database format</a>.
   *
   * @since 13.0
   */
  public static final MediaType MBOX = createConstant(APPLICATION_TYPE, "mbox");
  /**
   * <a href="http://goo.gl/1pGBFm">Apple over-the-air mobile configuration profiles</a>.