Fix SpotBugs errors

[ERROR] Medium:
org.apache.commons.io.input.ObservableInputStream.getObservers() may
expose internal representation by returning
ObservableInputStream.observers [org.apache.commons.io.input.ObservableInputStream]
At ObservableInputStream.java:[line 187] EI_EXPOSE_REP

Author: garydgregory

src/changes/changes.xml

@@ -67,6 +67,7 @@ The <action> type attribute can be add,update,fix,remove.
       <action dev="ggregory" type="fix" due-to="Gary Gregory">Fix SpotBugs error: org.apache.commons.io.IOExceptionList.getCauseList(Class) may expose internal representation by returning IOExceptionList.causeList [org.apache.commons.io.IOExceptionList] At IOExceptionList.java:[line 129] EI_EXPOSE_REP.</action>
       <action dev="ggregory" type="fix" due-to="Gary Gregory">Fix SpotBugs error: org.apache.commons.io.file.AccumulatorPathVisitor.getDirList() may expose internal representation by returning AccumulatorPathVisitor.dirList [org.apache.commons.io.file.AccumulatorPathVisitor] At AccumulatorPathVisitor.java:[line 179] EI_EXPOSE_REP.</action>
       <action dev="ggregory" type="fix" due-to="Gary Gregory">Fix SpotBugs error: org.apache.commons.io.file.AccumulatorPathVisitor.getFileList() may expose internal representation by returning AccumulatorPathVisitor.fileList [org.apache.commons.io.file.AccumulatorPathVisitor] At AccumulatorPathVisitor.java:[line 188] EI_EXPOSE_REP.</action>
+      <action dev="ggregory" type="fix" due-to="Gary Gregory">Fix SpotBugs error: org.apache.commons.io.input.ObservableInputStream.getObservers() may expose internal representation by returning ObservableInputStream.observers [org.apache.commons.io.input.ObservableInputStream] At ObservableInputStream.java:[line 187] EI_EXPOSE_REP.</action>
       <!-- UPDATE -->
       <action dev="ggregory" type="update" due-to="Gary Gregory">Bump org.codehaus.mojo:exec-maven-plugin from 3.1.0 to 3.1.1 #512.</action>
       <action dev="ggregory" type="update" due-to="Gary Gregory">Bump commons-lang3 from 3.13.0 to 3.14.0.</action>      

src/main/java/org/apache/commons/io/input/ObservableInputStream.java

@@ -178,13 +178,13 @@ private void forEachObserver(final IOConsumer<Observer> action) throws IOExcepti
     }
 
     /**
-     * Gets all currently registered observers.
+     * Gets a copy of currently registered observers.
      *
-     * @return a list of the currently registered observers.
+     * @return a copy of the list of currently registered observers.
      * @since 2.9.0
      */
     public List<Observer> getObservers() {
-        return observers;
+        return new ArrayList<>(observers);
     }
 
     /**