RFR: 8311097: Synchron XMLHttpRequest not receiving data

[Andrew Brygin]

This issue is a regression from  7e48413eb0 8285881: Update WebKit to 614.1. For synchronous requests, we have an additional buffer to collect data, and the appending data into this buffer is doing in a wrong way:  we use Vector::append(U&& u), which adds only the first data element. Instead, we should use Vector::append(const U* u, size_t size) in order to add all obtained data.

I have added the test for this issue to LoadTest.java

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Commit messages:
 - 8311097: Synchron XMLHttpRequest not receiving data

Changes: https://git.openjdk.org/jfx/pull/1227/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1227&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311097
  Stats: 52 lines in 4 files changed: 50 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1227.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1227/head:pull/1227

PR: https://git.openjdk.org/jfx/pull/1227