Judge null for key and value in attachment in RpcContextFilter.

[carryxyh]

Judge null for key and value in attachment in RpcContextFilter.
If not, NullPointerException will be thrown in RpcContextFilter in some case.

Fix #2127 : #2127 (comment)

[codecov-io]

Codecov Report

Merging #2171 into master will increase coverage by 0.28%.
The diff coverage is 44.44%.

@@             Coverage Diff              @@
##             master    #2171      +/-   ##
============================================
+ Coverage      54.3%   54.58%   +0.28%     
- Complexity     5111     5150      +39     
============================================
  Files           559      569      +10     
  Lines         24981    25036      +55     
  Branches       4453     4457       +4     
============================================
+ Hits          13566    13666     +100     
+ Misses         9380     9326      -54     
- Partials       2035     2044       +9

Impacted Files	Coverage Δ	Complexity Δ
...ache/dubbo/rpc/protocol/rest/RpcContextFilter.java	63.63% <44.44%> (-4.23%)	8 <1> (+1)
...n/java/org/apache/dubbo/common/utils/NetUtils.java	53.59% <0%> (-8.53%)	44% <0%> (ø)
...apache/dubbo/rpc/protocol/dubbo/FutureAdapter.java	58.06% <0%> (-6.46%)	3% <0%> (ø)
...rpc/protocol/dubbo/telnet/InvokeTelnetHandler.java	51.89% <0%> (-3.8%)	10% <0%> (-3%)
...he/dubbo/remoting/transport/netty/NettyServer.java	67.85% <0%> (-3.58%)	8% <0%> (-1%)
...bo/remoting/transport/netty/NettyCodecAdapter.java	53.12% <0%> (-1.57%)	3% <0%> (ø)
...dubbo/rpc/protocol/dubbo/CallbackServiceCodec.java	77.2% <0%> (-1.48%)	29% <0%> (ø)
.../dubbo/remoting/transport/netty4/NettyChannel.java	65% <0%> (-1.25%)	22% <0%> (-1%)
...apache/dubbo/rpc/protocol/dubbo/DubboProtocol.java	58.33% <0%> (-0.84%)	30% <0%> (-1%)
...he/dubbo/registry/multicast/MulticastRegistry.java	57.32% <0%> (-0.42%)	38% <0%> (-3%)
... and 16 more

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[diecui1202]

if ( (StringUtils.isNotEmpty(key) && illegalForRest(key)) || (StringUtils.isNotEmpty(value) && illegalForRest(value)) ) {
    throw new IllegalArgumentException("The attachments of " + RpcContext.class.getSimpleName() + " must not contain ',' or '=' when using rest protocol");
}

Is the above code better ?

[diecui1202]

Or judge null place here ?

[carryxyh]

Prefer judge null in 'illegalForRest'.
The meaning of this method is that if a value is not null, it should not contains ',' or '='.
I will fix this soon.
:)

[diecui1202]

LGTM.