Backport "MutablePropertySources#get throws when it should return null" [SPR-9185]

[spring-projects-issues]

Chris Beams opened SPR-9185 and commented

---

This issue is a backport sub-task of #13817

Referenced from: commits 7d72ab5

[spring-projects-issues]

Chris Beams commented

commit 7d72ab59ae1841fecb0f33c938b37234dde75a8f
Author: Chris Beams <cbeams@vmware.com>
Date:   Wed Feb 29 14:29:13 2012 +0100

    Return null correctly from MutablePropertySources#get
    
    Prior to this commit, MutablePropertySources#get(String) would throw
    IndexArrayOutOfBoundsException if the named property source does not
    actually exist. This is a violation of the PropertySource#get contract
    as described in its Javadoc.
    
    The implementation now correctly checks for the existence of the named
    property source, returning null if non-existent and otherwise returning
    the associated PropertySource.
    
    Other changes
    
     - Rename PropertySourcesTests => MutablePropertySourcesTests
     - Polish MutablePropertySourcesTests for style, formatting
     - Refactor MutablePropertySources for consistency
    
    Issue: SPR-9185
    Backport-Issue: SPR-9179
    Backport-Commit: 15d1d824b54b2ec36a7c926e12644b391b310930