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.
* <a href="https://en.wikipedia.org/wiki/RDF/XML">RDF/XML</a> documents, which are XML
   * serializations of <a
   * href="https://en.wikipedia.org/wiki/Resource_Description_Framework">Resource Description
   * Framework</a> graphs.
   *
   * @since 14.0
   */
  public static final MediaType RDF_XML_UTF_8 = createConstantUtf8(APPLICATION_TYPE, "rdf+xml");
  public static final MediaType RTF_UTF_8 = createConstantUtf8(APPLICATION_TYPE, "rtf");
  /**
   * SFNT fonts (which includes <a href="http://en.wikipedia.org/wiki/TrueType/">TrueType</a> and <a
   * href="http://en.wikipedia.org/wiki/OpenType/">OpenType</a> fonts). This is <a
   * href="http://www.iana.org/assignments/media-types/application/font-sfnt">registered</a> with
   * the IANA.
   *
   * @since 17.0
   */
  public static final MediaType SFNT = createConstant(APPLICATION_TYPE, "font-sfnt");
  public static final MediaType SHOCKWAVE_FLASH =
      createConstant(APPLICATION_TYPE, "x-shockwave-flash");
  /**
   * {@code skp} files produced by the 3D Modeling software <a
   * href="https://www.sketchup.com/">SketchUp</a>
   *
   * @since 13.0
   */
  public static final MediaType SKETCHUP = createConstant(APPLICATION_TYPE, "vnd.sketchup.skp");
  /**
   * As described in <a href="http://www.ietf.org/rfc/rfc3902.txt">RFC 3902</a>, this constant
   * ({@code application/soap+xml}) is used to identify SOAP 1.2 message envelopes that have been