Replace placeholders in snippet name as well as the operation name

Previously, StandardWriterResolver only replaced placeholders in the
operation name. This meant that the name and location of generated
snippets was unnecessarily limited.

This commit makes things more flexible by updating
StandardWriterResolver to replace placeholders in the snippet name
as well.

Closes gh-403

spring-restdocs-core/src/main/java/org/springframework/restdocs/snippet/StandardWriterResolver.java

@@ -82,11 +82,12 @@ public final class StandardWriterResolver implements WriterResolver {
 	@Override
 	public Writer resolve(String operationName, String snippetName,
 			RestDocumentationContext context) throws IOException {
-		File outputFile = resolveFile(
-				this.propertyPlaceholderHelper.replacePlaceholders(operationName,
-						this.placeholderResolverFactory.create(context)),
-				snippetName + "." + this.templateFormat.getFileExtension(), context);
-
+		PlaceholderResolver placeholderResolver = this.placeholderResolverFactory
+				.create(context);
+		String outputDirectory = replacePlaceholders(placeholderResolver, operationName);
+		String fileName = replacePlaceholders(placeholderResolver, snippetName) + "."
+				+ this.templateFormat.getFileExtension();
+		File outputFile = resolveFile(outputDirectory, fileName, context);
 		if (outputFile != null) {
 			createDirectoriesIfNecessary(outputFile);
 			return new OutputStreamWriter(new FileOutputStream(outputFile),
@@ -103,6 +104,10 @@ public final class StandardWriterResolver implements WriterResolver {
 		this.encoding = encoding;
 	}
 
+	private String replacePlaceholders(PlaceholderResolver resolver, String input) {
+		return this.propertyPlaceholderHelper.replacePlaceholders(input, resolver);
+	}
+
 	File resolveFile(String outputDirectory, String fileName,
 			RestDocumentationContext context) {
 		File outputFile = new File(outputDirectory, fileName);