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.
/*
 * Copyright (C) 2011 The Guava Authors
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 * Licensed under the Apache License, Version 2.0 (the "License");
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package com.google.common.net;
import static com.google.common.base.Charsets.UTF_16;
import static com.google.common.base.Charsets.UTF_8;
import static com.google.common.net.MediaType.ANY_APPLICATION_TYPE;
import static com.google.common.net.MediaType.ANY_AUDIO_TYPE;
import static com.google.common.net.MediaType.ANY_IMAGE_TYPE;
import static com.google.common.net.MediaType.ANY_TEXT_TYPE;
import static com.google.common.net.MediaType.ANY_TYPE;
import static com.google.common.net.MediaType.ANY_VIDEO_TYPE;
import static com.google.common.net.MediaType.HTML_UTF_8;