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.
long skipped = skipSafely(in, remaining);
      if (skipped == 0) {
        
        
        int skip = (int) Math.min(remaining, buf.length);
        if ((skipped = in.read(buf, 0, skip)) == -1) {
          
          break;
        }
      }
      totalSkipped += skipped;
    }
    return totalSkipped;
  }
  /**
   * Attempts to skip up to {@code n} bytes from the given input stream, but not more than {@code
   * in.available()} bytes. This prevents {@code FileInputStream} from skipping more bytes than
   * actually remain in the file, something that it {@linkplain java.io.FileInputStream#skip(long)
   * specifies} it can do in its Javadoc despite the fact that it is violating the contract of
   * {@code InputStream.skip()}.
   */
  private static long skipSafely(InputStream in, long n) throws IOException {
    int available = in.available();
    return available == 0 ? 0 : in.skip(Math.min(available, n));
  }
  /**
   * Process the bytes of the given input stream using the given processor.
   *
   * @param input the input stream to process
   * @param processor the object to which to pass the bytes of the stream