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.
.addEqualityGroup(
            MediaType.create("text", "plain").withCharset(UTF_8),
            MediaType.create("text", "plain").withParameter("CHARSET", "UTF-8"),
            MediaType.create("text", "plain")
                .withParameters(ImmutableMultimap.of("charset", "utf-8")),
            MediaType.parse("text/plain;charset=utf-8"),
            MediaType.parse("text/plain; charset=utf-8"),
            MediaType.parse("text/plain;  charset=utf-8"),
            MediaType.parse("text/plain; \tcharset=utf-8"),
            MediaType.parse("text/plain; \r\n\tcharset=utf-8"),
            MediaType.parse("text/plain; CHARSET=utf-8"),
            MediaType.parse("text/plain; charset=\"utf-8\""),
            MediaType.parse("text/plain; charset=\"\\u\\tf-\\8\""),
            MediaType.parse("text/plain; charset=UTF-8"),
            MediaType.parse("text/plain ; charset=utf-8"))
        .addEqualityGroup(MediaType.parse("text/plain; charset=utf-8; charset=utf-8"))
        .addEqualityGroup(
            MediaType.create("text", "plain").withParameter("a", "value"),
            MediaType.create("text", "plain").withParameter("A", "value"))
        .addEqualityGroup(
            MediaType.create("text", "plain").withParameter("a", "VALUE"),
            MediaType.create("text", "plain").withParameter("A", "VALUE"))
        .addEqualityGroup(
            MediaType.create("text", "plain")
                .withParameters(ImmutableListMultimap.of("a", "1", "a", "2")),
            MediaType.create("text", "plain")
                .withParameters(ImmutableListMultimap.of("a", "2", "a", "1")))
        .addEqualityGroup(MediaType.create("text", "csv"))
        .addEqualityGroup(MediaType.create("application", "atom+xml"))
        .testEquals();