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.
* beginning.
   */
  @Beta
  public static ByteArrayDataInput newDataInput(byte[] bytes) {
    return newDataInput(new ByteArrayInputStream(bytes));
  }
  /**
   * Returns a new {@link ByteArrayDataInput} instance to read from the {@code bytes} array,
   * starting at the given position.
   *
   * @throws IndexOutOfBoundsException if {@code start} is negative or greater than the length of
   *     the array
   */
  @Beta
  public static ByteArrayDataInput newDataInput(byte[] bytes, int start) {
    checkPositionIndex(start, bytes.length);
    return newDataInput(new ByteArrayInputStream(bytes, start, bytes.length - start));
  }
  /**
   * Returns a new {@link ByteArrayDataInput} instance to read from the given {@code
   * ByteArrayInputStream}. The given input stream is not reset before being read from by the
   * returned {@code ByteArrayDataInput}.
   *
   * @since 17.0
   */
  @Beta
  public static ByteArrayDataInput newDataInput(ByteArrayInputStream byteArrayInputStream) {
    return new ByteArrayDataInputStream(checkNotNull(byteArrayInputStream));
  }
  private static class ByteArrayDataInputStream implements ByteArrayDataInput {