Fix IndexOutOfBoundsException when calling PostRequest.getUpstreamHeaders

[MariaLapovska]

Please answer these questions before submitting a pull request

• Why submit this pull request?

• Bugfix

• New feature provided

• Improve performance

• Related issues

• bug: PostRequest.getUpstreamHeaders cause IndexOutOfBoundsException Error #325

• request help: request.getUpstreamHeaders java.lang.IndexOutOfBoundsException: null? #228

• bug: 在PluginFilter类中通过requiredRespBody()方法设置为true后，在插件中调用PostRequest..getUpstreamHeaders()方法会偶现(概率很高)报错java.lang.IndexOutOfBoundsException: null #234

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Bugfix

• Description
  Currently, if you try to call PostRequest.getUpstreamHeaders() in postFilter(), it intermittently throws IndexOutOfBoundsException (specifically headersLength() method in Req class):

java.lang.IndexOutOfBoundsException: null
        at java.base/java.nio.Buffer$1.apply(Buffer.java:757) ~[na:na]
        at java.base/java.nio.Buffer$1.apply(Buffer.java:754) ~[na:na]
        at java.base/jdk.internal.util.Preconditions$4.apply(Preconditions.java:213) ~[na:na]
        at java.base/jdk.internal.util.Preconditions$4.apply(Preconditions.java:210) ~[na:na]
        at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:98) ~[na:na]
        at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:106) ~[na:na]
        at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:302) ~[na:na]
        at java.base/java.nio.Buffer.checkIndex(Buffer.java:779) ~[na:na]
        at java.base/java.nio.DirectByteBuffer.getInt(DirectByteBuffer.java:761) ~[na:na]
        at com.google.flatbuffers.Table.__vector_len(Table.java:125) ~[flatbuffers-java-2.0.0.jar!/:na]
        at io.github.api7.A6.HTTPRespCall.Req.headersLength(Req.java:22) ~[A6-0.6.0-RELEASE.jar!/:na]
        at org.apache.apisix.plugin.runner.PostRequest.getUpstreamHeaders(PostRequest.java:82) ~[apisix-runner-plugin-sdk-0.6.0.jar!/:0.6.0]
        at gg.lita.arch.gateway.plugin.filters.LitaLoginResponseFilter.getPostRequestHeaders(LitaLoginResponseFilter.java:52) ~[!/:0.0.1]

• How to fix?
  If we follow the same pattern as RpcHandler does with HttpRequest, where it pre-reads the headers, we avoid the given issue, where by the time PostRequest.getUpstreamHeaders() is called from inside one of the plugins, the ByteBuffer could be corrupted, reused, or have a wrong position.