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 14.0
   */
  public static final MediaType APPLICATION_XML_UTF_8 = createConstantUtf8(APPLICATION_TYPE, "xml");
  public static final MediaType ATOM_UTF_8 = createConstantUtf8(APPLICATION_TYPE, "atom+xml");
  public static final MediaType BZIP2 = createConstant(APPLICATION_TYPE, "x-bzip2");
  /**
   * Files in the <a href="https://www.dartlang.org/articles/embedding-in-html/">dart</a>
   * programming language.
   *
   * @since 19.0
   */
  public static final MediaType DART_UTF_8 = createConstantUtf8(APPLICATION_TYPE, "dart");
  /**
   * <a href="https://goo.gl/2QoMvg">Apple Passbook</a>.
   *
   * @since 19.0
   */
  public static final MediaType APPLE_PASSBOOK =
      createConstant(APPLICATION_TYPE, "vnd.apple.pkpass");
  /**
   * <a href="http://en.wikipedia.org/wiki/Embedded_OpenType">Embedded OpenType</a> fonts. This is
   * <a href="http://www.iana.org/assignments/media-types/application/vnd.ms-fontobject">registered
   * </a> with the IANA.
   *
   * @since 17.0
   */
  public static final MediaType EOT = createConstant(APPLICATION_TYPE, "vnd.ms-fontobject");
  /**
   * As described in the <a href="http://idpf.org/epub">International Digital Publishing Forum</a>