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.
}
  @GwtIncompatible // Non-UTF-8 Charset
  public void testEquals_nonUtf8Charsets() {
    new EqualsTester()
        .addEqualityGroup(MediaType.create("text", "plain"))
        .addEqualityGroup(MediaType.create("text", "plain").withCharset(UTF_8))
        .addEqualityGroup(MediaType.create("text", "plain").withCharset(UTF_16))
        .testEquals();
  }
  @GwtIncompatible // com.google.common.testing.NullPointerTester
  public void testNullPointer() {
    NullPointerTester tester = new NullPointerTester();
    tester.testAllPublicConstructors(MediaType.class);
    tester.testAllPublicStaticMethods(MediaType.class);
    tester.testAllPublicInstanceMethods(MediaType.parse("text/plain"));
  }
  public void testToString() {
    assertEquals("text/plain", MediaType.create("text", "plain").toString());
    assertEquals(
        "text/plain; something=\"cr@zy\"; something-else=\"crazy with spaces\";"
            + " and-another-thing=\"\"; normal-thing=foo",
        MediaType.create("text", "plain")
            .withParameter("something", "cr@zy")
            .withParameter("something-else", "crazy with spaces")
            .withParameter("and-another-thing", "")
            .withParameter("normal-thing", "foo")
            .toString());
  }
}