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.
public void testConstants_areUnique() {
    assertThat(getConstants()).containsNoDuplicates();
  }
  @GwtIncompatible 
  private static FluentIterable<Field> getConstantFields() {
    return FluentIterable.from(asList(MediaType.class.getDeclaredFields()))
        .filter(
            new Predicate<Field>() {
              @Override
              public boolean apply(Field input) {
                int modifiers = input.getModifiers();
                return isPublic(modifiers)
                    && isStatic(modifiers)
                    && isFinal(modifiers)
                    && MediaType.class.equals(input.getType());
              }
            });
  }
  @GwtIncompatible 
  private static FluentIterable<MediaType> getConstants() {
    return getConstantFields()
        .transform(
            new Function<Field, MediaType>() {
              @Override
              public MediaType apply(Field input) {
                try {
                  return (MediaType) input.get(null);
                } catch (Exception e) {
                  throw new RuntimeException(e);
                }