Lowercase all stage names

This avoids weird situations where someone tries to work around stage
sorting by relying on where lower vs upper case sort, or try to bypass
final or snapshot behavior by using a different case.

reckon-core/src/main/java/org/ajoberstar/reckon/core/Reckoner.java

@@ -100,6 +100,7 @@ private Version reckonTargetVersion(VcsInventory inventory, Version targetNormal
     String stage = stageCalc.apply(inventory, targetNormal)
         .map(String::trim)
         .filter(s -> !s.isEmpty())
+        .map(String::toLowerCase)
         .orElse(null);
 
     if (stage != null && !stages.contains(stage)) {
@@ -191,7 +192,9 @@ public Builder scopeCalc(Function<VcsInventory, Optional<String>> scopeCalc) {
      * @return this builder
      */
     public Builder stages(String... stages) {
-      this.stages = Arrays.stream(stages).collect(Collectors.toSet());
+      this.stages = Arrays.stream(stages)
+          .map(String::toLowerCase)
+          .collect(Collectors.toSet());
       this.defaultStage = this.stages.stream()
           .filter(name -> !FINAL_STAGE.equals(name))
           .sorted()

reckon-core/src/test/groovy/org/ajoberstar/reckon/core/ReckonerTest.groovy

@@ -18,6 +18,34 @@ class ReckonerTest extends Specification {
     thrown(IllegalArgumentException)
   }
 
+  @Unroll
+  def 'stages are lowercased'(String stage) {
+    given:
+    VcsInventory inventory = new VcsInventory(
+      'abcdef',
+      true,
+      null,
+      Version.valueOf('1.2.3-beta.1'),
+      Version.valueOf('1.2.2'),
+      1,
+      [Version.valueOf('1.3.0')] as Set,
+      [Version.valueOf('2.0.0-rc.1')] as Set
+    )
+    when:
+    Reckoner.builder()
+      .clock(CLOCK)
+      .vcs { -> inventory }
+      .scopeCalc { i -> Optional.empty() }
+      .stages('beTA', 'miLEStone', 'RC', 'Final')
+      .stageCalc { i, v -> Optional.ofNullable(stage) }
+      .build()
+      .reckon()
+    then:
+    notThrown(IllegalArgumentException)
+    where:
+    stage << ['BeTa', 'Milestone', 'rc', 'fINal']
+  }
+
   def 'if version is claimed, throw'() {
     given:
     VcsInventory inventory = new VcsInventory(