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.
* Media type for the <a href="https://tools.ietf.org/html/rfc7946">GeoJSON Format</a>, a
   * geospatial data interchange format based on JSON.
   *
   * @since 28.0
   */
  public static final MediaType GEO_JSON = createConstant(APPLICATION_TYPE, "geo+json");
  public static final MediaType GZIP = createConstant(APPLICATION_TYPE, "x-gzip");
  /**
   * <a href="https://tools.ietf.org/html/draft-kelly-json-hal-08#section-3">JSON Hypertext
   * Application Language (HAL) documents</a>.
   *
   * @since 26.0
   */
  public static final MediaType HAL_JSON = createConstant(APPLICATION_TYPE, "hal+json");
  /**
   * <a href="http://www.rfc-editor.org/rfc/rfc4329.txt">RFC 4329</a> declares this to be the
   * correct media type for JavaScript, but {@link #TEXT_JAVASCRIPT_UTF_8 text/javascript} may be
   * necessary in certain situations for compatibility.
   */
  public static final MediaType JAVASCRIPT_UTF_8 =
      createConstantUtf8(APPLICATION_TYPE, "javascript");
  /**
   * For <a href="https://tools.ietf.org/html/rfc7515">JWS or JWE objects using the Compact
   * Serialization</a>.
   *
   * @since 27.1
   */
  public static final MediaType JOSE = createConstant(APPLICATION_TYPE, "jose");
  /**
   * For <a href="https://tools.ietf.org/html/rfc7515">JWS or JWE objects using the JSON