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.
"reached end of stream after reading " + read + " bytes; " + len + " bytes expected");
    }
  }
  /**
   * Discards {@code n} bytes of data from the input stream. This method will block until the full
   * amount has been skipped. Does not close the stream.
   *
   * @param in the input stream to read from
   * @param n the number of bytes to skip
   * @throws EOFException if this stream reaches the end before skipping all the bytes
   * @throws IOException if an I/O error occurs, or the stream does not support skipping
   */
  @Beta
  public static void skipFully(InputStream in, long n) throws IOException {
    long skipped = skipUpTo(in, n);
    if (skipped < n) {
      throw new EOFException(
          "reached end of stream after skipping " + skipped + " bytes; " + n + " bytes expected");
    }
  }
  /**
   * Discards up to {@code n} bytes of data from the input stream. This method will block until
   * either the full amount has been skipped or until the end of the stream is reached, whichever
   * happens first. Returns the total number of bytes skipped.
   */
  static long skipUpTo(InputStream in, final long n) throws IOException {
    long totalSkipped = 0;
    byte[] buf = createBuffer();
    while (totalSkipped < n) {
      long remaining = n - totalSkipped;