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.
*
   * @throws IllegalArgumentException if subtype is invalid
   */
  static MediaType createVideoType(String subtype) {
    return create(VIDEO_TYPE, subtype);
  }
  private static String normalizeToken(String token) {
    checkArgument(TOKEN_MATCHER.matchesAllOf(token));
    checkArgument(!token.isEmpty());
    return Ascii.toLowerCase(token);
  }
  private static String normalizeParameterValue(String attribute, String value) {
    checkNotNull(value); 
    checkArgument(ascii().matchesAllOf(value), "parameter values must be ASCII: %s", value);
    return CHARSET_ATTRIBUTE.equals(attribute) ? Ascii.toLowerCase(value) : value;
  }
  /**
   * Parses a media type from its string representation.
   *
   * @throws IllegalArgumentException if the input is not parsable
   */
  public static MediaType parse(String input) {
    checkNotNull(input);
    Tokenizer tokenizer = new Tokenizer(input);
    try {
      String type = tokenizer.consumeToken(TOKEN_MATCHER);
      tokenizer.consumeCharacter('/');
      String subtype = tokenizer.consumeToken(TOKEN_MATCHER);
      ImmutableListMultimap.Builder<String, String> parameters = ImmutableListMultimap.builder();
      while (tokenizer.hasMore()) {