Fix uncompilable code of Java asynchttp

[wtetsu]

I noticed, java/asynchttp/custom-method.java is not compilable because there is no preparePropfind method.

AsyncHttpClientclient = newDefaultAsyncHttpClient();
client.preparePropfind("http://mockbin.com/har")

I unified code style from client.prepareMethod(url) to client.prepare("METHOD", url) because internal code of DefaultAsyncHttpClient is like this.

publicBoundRequestBuilderprepare(Stringmethod, Stringurl) {
returnthis.requestBuilder(method, url);
}
publicBoundRequestBuilderprepareGet(Stringurl) {
returnthis.requestBuilder("GET", url);
}
publicBoundRequestBuilderprepareConnect(Stringurl) {
returnthis.requestBuilder("CONNECT", url);
}
publicBoundRequestBuilderprepareOptions(Stringurl) {
returnthis.requestBuilder("OPTIONS", url);
}
publicBoundRequestBuilderprepareHead(Stringurl) {
returnthis.requestBuilder("HEAD", url);
}
publicBoundRequestBuilderpreparePost(Stringurl) {
returnthis.requestBuilder("POST", url);
}
publicBoundRequestBuilderpreparePut(Stringurl) {
returnthis.requestBuilder("PUT", url);
}
// ...