Fix error in sample-rate minimum of BoundrySampler. (openzipkin#188)

Force usage of float in float-comparison, 0.0001f >= 0.0001 evaluates to false.
Fix docs-error in Sampler to reflect change in default samplers made in Brave 3.6.1

brave-core/src/main/java/com/github/kristofa/brave/BoundarySampler.java

@@ -28,7 +28,7 @@ public final class BoundarySampler extends Sampler {
   public static Sampler create(float rate) {
     if (rate == 0) return Sampler.NEVER_SAMPLE;
     if (rate == 1.0) return ALWAYS_SAMPLE;
-    checkArgument(rate > 0.0001 && rate < 1, "rate should be between 0.0001 and 1: was %s", rate);
+    checkArgument(rate >= 0.0001f && rate < 1, "rate should be between 0.0001 and 1: was %s", rate);
     final long boundary = (long) (rate * 10000); // safe cast as less <= 1
     return new BoundarySampler(boundary);
   }

brave-core/src/main/java/com/github/kristofa/brave/Sampler.java

@@ -43,7 +43,7 @@ public abstract class Sampler {
    * <p>The sampler returned is good for low volumes of traffic (<100K requests), as it is precise.
    * If you have high volumes of traffic, consider {@link BoundarySampler}.
    *
-   * @param rate minimum sample rate is 0.0001, or 0.01% of traces
+   * @param rate minimum sample rate is 0.01, or 1% of traces
    */
   public static Sampler create(float rate) {
     return CountingSampler.create(rate);

brave-core/src/test/java/com/github/kristofa/brave/BoundarySamplerTest.java

@@ -1,6 +1,7 @@
 package com.github.kristofa.brave;
 
 import org.assertj.core.data.Percentage;
+import org.junit.Test;
 
 import static org.assertj.core.data.Percentage.withPercentage;
 
@@ -12,4 +13,9 @@ public class BoundarySamplerTest extends SamplerTest {
   @Override Percentage expectedErrorRate() {
     return withPercentage(10);
   }
+
+  @Test
+  public void acceptsOneInTenThousandSampleRate() {
+    newSampler(0.0001f);
+  }
 }

brave-core/src/test/java/com/github/kristofa/brave/CountingSamplerTest.java

@@ -1,6 +1,8 @@
 package com.github.kristofa.brave;
 
 import org.assertj.core.data.Percentage;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
 
 import static org.assertj.core.data.Percentage.withPercentage;
 
@@ -12,4 +14,13 @@ public class CountingSamplerTest extends SamplerTest {
   @Override Percentage expectedErrorRate() {
     return withPercentage(0);
   }
+
+
+  @Test
+  public void sampleRateMinimumOnePercent() throws Exception {
+    thrown.expect(IllegalArgumentException.class);
+    newSampler(0.0001f);
+  }
+
+
 }