Work around mockwebserver hangs in S3 tests

Previously S3ClientMockTest.testZeroLengthPutHasContentLengthHeader
would hang in MWS at:

```
at java.io.BufferedInputStream.read(BufferedInputStream.java:265)
- locked <0x00000000d6d8fd60> (a java.io.BufferedInputStream)
at com.squareup.okhttp.mockwebserver.MockWebServer.readAsciiUntilCrlf(MockWebServer.java:570)
at com.squareup.okhttp.mockwebserver.MockWebServer.readRequest(MockWebServer.java:439)
```

and in jclouds at:

```
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1439)
- locked <0x00000000d6d012e0> (a sun.net.www.protocol.http.HttpURLConnection)
at org.jclouds.http.internal.JavaUrlHttpCommandExecutorService.invoke(JavaUrlHttpCommandExecutorService.java:103)
at org.jclouds.http.internal.JavaUrlHttpCommandExecutorService.invoke(JavaUrlHttpCommandExecutorService.java:71)
```