Upgrade to Kotlin 1.1.50

Potential regression detected for null-safety on user project side
https://youtrack.jetbrains.com/issue/KT-20411

build.gradle

@@ -14,7 +14,7 @@ buildscript {
 plugins {
 	id "com.gradle.build-scan" version "1.8"
 	id "io.spring.dependency-management" version "1.0.3.RELEASE" apply false
-	id "org.jetbrains.kotlin.jvm" version "1.1.4-3" apply false
+	id "org.jetbrains.kotlin.jvm" version "1.1.50" apply false
 	id "org.jetbrains.dokka" version "0.9.15"
 	id "org.asciidoctor.convert" version "1.5.3"
 }
@@ -50,7 +50,7 @@ configure(allprojects) { project ->
 	ext.junitJupiterVersion  = '5.0.0'
 	ext.junitPlatformVersion = '1.0.0'
 	ext.junitVintageVersion  = "4.12.0"
-	ext.kotlinVersion        = '1.1.4-3'
+	ext.kotlinVersion        = '1.1.50'
 	ext.log4jVersion         = '2.9.1'
 	ext.nettyVersion         = "4.1.15.Final"
 	ext.reactorVersion       = "Bismuth-RC1"
@@ -72,12 +72,12 @@ configure(allprojects) { project ->
 	apply plugin: "kotlin"
 	compileKotlin {
 		kotlinOptions.jvmTarget = "1.8"
-		kotlinOptions.freeCompilerArgs = ["-Xskip-runtime-version-check", "-Xjsr305-annotations=enable"]
+		kotlinOptions.freeCompilerArgs = ["-Xskip-runtime-version-check", "-Xjsr305=strict"]
 
 	}
 	compileTestKotlin {
 		kotlinOptions.jvmTarget = "1.8"
-		kotlinOptions.freeCompilerArgs = ["-Xskip-runtime-version-check", "-Xjsr305-annotations=enable"]
+		kotlinOptions.freeCompilerArgs = ["-Xskip-runtime-version-check", "-Xjsr305=strict"]
 	}
 
 	configurations.all {

src/docs/asciidoc/kotlin.adoc

@@ -105,9 +105,12 @@ Other libraries like Reactor and Spring Data leverage these annotations as well
 null-safe APIs for Kotlin developers.
 ====
 
-For now, one needs to use a `-Xjsr305-annotations=enable` flag (specified via the
-`freeCompilerArgs` property with Maven or Gradle Kotlin plugins), but that should become
-the default behavior in an upcoming release of Kotlin.
+The JSR 305 checks can be configured by adding the `-Xjsr305` compiler flag with the following
+options: `-Xjsr305={strict|warn|ignore}`.
+
+For kotlin versions 1.1.50+/1.2, the default behavior is the same to `-Xjsr305=warn`. The
+`strict` value should be considered experimental (Spring API nullability could evolve and
+more checks may be added in the future).
 
 [NOTE]
 ====