fix issue-2948:spring boot external config invalid

[meiping05]

Environment

• Dubbo version: 2.6.0
• Operating System version: Red Hat Enterprise Linux Server release 6.3
• Java version: 1.8.0_92

Steps to reproduce this issue

Spring Boot使用外部配置文件,没有起作用,还是使用的包内application.properties

[codecov-io]

Codecov Report

Merging #2950 into master will decrease coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2950      +/-   ##
==========================================
- Coverage   64.04%   63.98%   -0.06%     
==========================================
  Files         584      584              
  Lines       26050    26050              
  Branches     4562     4562              
==========================================
- Hits        16683    16668      -15     
- Misses       7190     7208      +18     
+ Partials     2177     2174       -3

Impacted Files	Coverage Δ
...dubbo/config/spring/util/PropertySourcesUtils.java	85.71% <100%> (ø)	⬆️
...che/dubbo/remoting/transport/mina/MinaChannel.java	43.42% <0%> (-10.53%)	⬇️
...e/dubbo/remoting/transport/netty/NettyChannel.java	57.64% <0%> (-4.71%)	⬇️
...dubbo/remoting/exchange/support/DefaultFuture.java	67.78% <0%> (-3.36%)	⬇️
...dubbo/rpc/protocol/dubbo/CallbackServiceCodec.java	77.2% <0%> (-2.21%)	⬇️
...rpc/protocol/dubbo/telnet/InvokeTelnetHandler.java	70.17% <0%> (-1.76%)	⬇️
...he/dubbo/registry/multicast/MulticastRegistry.java	63.79% <0%> (-1.73%)	⬇️
...apache/dubbo/rpc/protocol/dubbo/DubboProtocol.java	65.83% <0%> (-0.84%)	⬇️
...pache/dubbo/registry/support/AbstractRegistry.java	81.2% <0%> (-0.76%)	⬇️
.../java/org/apache/dubbo/config/ReferenceConfig.java	55.68% <0%> (-0.38%)	⬇️
... and 6 more

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