Fix Gradle build to read files in UTF-8

https://build.spring.io/browse/INT-SI51X-18

When we have non-standard symbols in the source code, the `updateCopyrights`
Gradle task may break them after because of non-compatible encoding
between file reader and source code.

* Add `org.gradle.jvmargs='-Dfile.encoding=UTF-8'` to the `gradle.properties`
to enforce an expected file reading encoding.
* Fix `AbstractFilePayloadTransformerTests` to bring non-standard
symbols back

build.gradle

@@ -232,7 +232,7 @@ subprojects { subproject ->
 								def years = "$beginningYear-$now"
 								def sourceCode = file.text
 								sourceCode = sourceCode.replaceFirst(/20\d\d(-20\d\d)?/, years)
-								file.write(sourceCode)
+								file.text = sourceCode
 								println "Copyright updated for file: $file"
 							}
 							break

gradle.properties

@@ -1 +1,2 @@
 version=5.1.4.BUILD-SNAPSHOT
+org.gradle.jvmargs='-Dfile.encoding=UTF-8'

spring-integration-file/src/test/java/org/springframework/integration/file/transformer/AbstractFilePayloadTransformerTests.java

@@ -42,7 +42,7 @@ public abstract class AbstractFilePayloadTransformerTests<T extends AbstractFile
 
 	static final String DEFAULT_ENCODING = "UTF-8";
 
-	static final String SAMPLE_CONTENT = "HelloWorld\n????";
+	static final String SAMPLE_CONTENT = "HelloWorld\näöüß";
 
 	T transformer;