Fix Quantity hashcode method. (typelevel#307)

* Fix Quantity hashcode method. Previous version was broken equals/hashcode conduct.

* Add hashcode method tests.

shared/src/main/scala/squants/Quantity.scala

@@ -8,6 +8,8 @@
 
 package squants
 
+import java.util.Objects
+
 import scala.math.BigDecimal.RoundingMode
 import scala.math.BigDecimal.RoundingMode.RoundingMode
 
@@ -176,7 +178,9 @@ abstract class Quantity[A <: Quantity[A]] extends Serializable with Ordered[A] {
    *
    * @return
    */
-  override def hashCode() = toString.hashCode()
+  override def hashCode() = {
+    Objects.hash(dimension, Double.box(to(dimension.primaryUnit)))
+  }
 
   /**
    * Returns boolean result of approximate equality comparison

shared/src/test/scala/squants/QuantitySpec.scala

@@ -686,4 +686,20 @@ class QuantitySpec extends FlatSpec with Matchers with CustomMatchers with TryVa
     m.failure.exception shouldBe a[QuantityParseException]
     m.failure.exception should have message("Unable to identify Mass unit m:(100.0,m)")
   }
+
+  it should "return consistent hashcode" in {
+    val timeInMinutes = Minutes(1)
+
+    timeInMinutes.hashCode() shouldBe timeInMinutes.hashCode()
+  }
+
+  it should "return equal hashcode, when objects are equal" in {
+
+    val timeInMinutes = Minutes(1)
+    val timeInSeconds = Seconds(60)
+
+    timeInMinutes.equals(timeInSeconds) shouldBe true
+    timeInMinutes.hashCode() shouldBe timeInSeconds.hashCode()
+
+  }
 }