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.
while (buf.hasRemaining()) {
        total += to.write(buf);
      }
      buf.clear();
    }
    return total;
  }
  
  private static final int MAX_ARRAY_LEN = Integer.MAX_VALUE - 8;
  
  private static final int TO_BYTE_ARRAY_DEQUE_SIZE = 20;
  /**
   * Returns a byte array containing the bytes from the buffers already in {@code bufs} (which have
   * a total combined length of {@code totalLen} bytes) followed by all bytes remaining in the given
   * input stream.
   */
  private static byte[] toByteArrayInternal(InputStream in, Deque<byte[]> bufs, int totalLen)
      throws IOException {
    
    
    
    for (int bufSize = BUFFER_SIZE;
        totalLen < MAX_ARRAY_LEN;
        bufSize = IntMath.saturatedMultiply(bufSize, 2)) {
      byte[] buf = new byte[Math.min(bufSize, MAX_ARRAY_LEN - totalLen)];
      bufs.add(buf);
      int off = 0;
      while (off < buf.length) {
        
        int r = in.read(buf, off, buf.length - off);