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.
* @return the result of the byte processor
   * @throws IOException if an I/O error occurs
   * @since 14.0
   */
  @Beta
  @CanIgnoreReturnValue 
  public static <T> T readBytes(InputStream input, ByteProcessor<T> processor) throws IOException {
    checkNotNull(input);
    checkNotNull(processor);
    byte[] buf = createBuffer();
    int read;
    do {
      read = input.read(buf);
    } while (read != -1 && processor.processBytes(buf, 0, read));
    return processor.getResult();
  }
  /**
   * Reads some bytes from an input stream and stores them into the buffer array {@code b}. This
   * method blocks until {@code len} bytes of input data have been read into the array, or end of
   * file is detected. The number of bytes read is returned, possibly zero. Does not close the
   * stream.
   *
   * <p>A caller can detect EOF if the number of bytes read is less than {@code len}. All subsequent
   * calls on the same stream will return zero.
   *
   * <p>If {@code b} is null, a {@code NullPointerException} is thrown. If {@code off} is negative,
   * or {@code len} is negative, or {@code off+len} is greater than the length of the array {@code
   * b}, then an {@code IndexOutOfBoundsException} is thrown. If {@code len} is zero, then no bytes
   * are read. Otherwise, the first byte read is stored into element {@code b[off]}, the next one
   * into {@code b[off+1]}, and so on. The number of bytes read is, at most, equal to {@code len}.