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.
/**
   * WASM applications. For more information see <a href="https://webassembly.org/">the Web Assembly
   * overview</a>.
   *
   * @since 27.0
   */
  public static final MediaType WASM_APPLICATION = createConstant(APPLICATION_TYPE, "wasm");
  /**
   * NaCl applications. For more information see <a
   * href="https://developer.chrome.com/native-client/devguide/coding/application-structure">the
   * Developer Guide for Native Client Application Structure</a>.
   *
   * @since 20.0
   */
  public static final MediaType NACL_APPLICATION = createConstant(APPLICATION_TYPE, "x-nacl");
  /**
   * NaCl portable applications. For more information see <a
   * href="https://developer.chrome.com/native-client/devguide/coding/application-structure">the
   * Developer Guide for Native Client Application Structure</a>.
   *
   * @since 20.0
   */
  public static final MediaType NACL_PORTABLE_APPLICATION =
      createConstant(APPLICATION_TYPE, "x-pnacl");
  public static final MediaType OCTET_STREAM = createConstant(APPLICATION_TYPE, "octet-stream");
  public static final MediaType OGG_CONTAINER = createConstant(APPLICATION_TYPE, "ogg");
  public static final MediaType OOXML_DOCUMENT =
      createConstant(
          APPLICATION_TYPE, "vnd.openxmlformats-officedocument.wordprocessingml.document");
  public static final MediaType OOXML_PRESENTATION =