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.
assertTrue(ANY_TYPE.hasWildcard());
    assertTrue(ANY_APPLICATION_TYPE.hasWildcard());
    assertTrue(ANY_AUDIO_TYPE.hasWildcard());
    assertTrue(ANY_IMAGE_TYPE.hasWildcard());
    assertTrue(ANY_TEXT_TYPE.hasWildcard());
    assertTrue(ANY_VIDEO_TYPE.hasWildcard());
  }
  public void testIs() {
    assertTrue(PLAIN_TEXT_UTF_8.is(ANY_TYPE));
    assertTrue(JPEG.is(ANY_TYPE));
    assertTrue(ANY_TEXT_TYPE.is(ANY_TYPE));
    assertTrue(PLAIN_TEXT_UTF_8.is(ANY_TEXT_TYPE));
    assertTrue(PLAIN_TEXT_UTF_8.withoutParameters().is(ANY_TEXT_TYPE));
    assertFalse(JPEG.is(ANY_TEXT_TYPE));
    assertTrue(PLAIN_TEXT_UTF_8.is(PLAIN_TEXT_UTF_8));
    assertTrue(PLAIN_TEXT_UTF_8.is(PLAIN_TEXT_UTF_8.withoutParameters()));
    assertFalse(PLAIN_TEXT_UTF_8.withoutParameters().is(PLAIN_TEXT_UTF_8));
    assertFalse(PLAIN_TEXT_UTF_8.is(HTML_UTF_8));
    assertFalse(PLAIN_TEXT_UTF_8.withParameter("charset", "UTF-16").is(PLAIN_TEXT_UTF_8));
    assertFalse(PLAIN_TEXT_UTF_8.is(PLAIN_TEXT_UTF_8.withParameter("charset", "UTF-16")));
  }
  public void testParse_empty() {
    try {
      MediaType.parse("");
      fail();
    } catch (IllegalArgumentException expected) {
    }
  }
  public void testParse_badInput() {
    try {