[Spring] fix <async>false</async> in the Maven plugin

Created by: raghuraman1

Hi, I am using the openapi-generator-maven-plugin with these versions: 3.3.1-SNAPSHOT (thats master ). I came across this bug. Please see code snippet below of the pom.xml alongwith my suggestions for the fix..

I have also noted another smaller issue unrelated to this. Thanks. R Please note value of asynch: false.

            <plugin>
                          <groupId>org.openapitools</groupId>
			<artifactId>openapi-generator-maven-plugin</artifactId>
			<version>${openapi.codegen.version}</version>
			<executions>
				<execution>
					<id>common</id>
					<goals>
						<goal>generate</goal>
					</goals>
					<configuration>
                                              <inputSpec>${project.basedir}/src/main/resources/api.yaml</inputSpec>
						<modelPackage>com.example.api.models</modelPackage>
						<apiPackage>com.example.api</apiPackage>
						<output>${project.basedir}</output>					
						<language>spring</language>
						<invokerPackage>com.example.api</invokerPackage>
						<basePackage>com.example.api</basePackage>
						<withXml>true</withXml>
						<configOptions>
						
							<artifactId>foo</artifactId>
							<artifactDescription>Test API</artifactDescription>
							<title>Test API</title>
							<artifactUrl>https://api.example.com</artifactUrl>
							<groupId>com.example.api</groupId>
							<artifactVersion>1</artifactVersion>								
							<sourceFolder>src/gen/java/main</sourceFolder>
							<serializableModel>true</serializableModel>
							<dateLibrary>java8</dateLibrary>
							<java8>true</java8>
							<async>false</async>
							<library>spring-boot</library>		
							<delegatePattern>true</delegatePattern>
							<useBeanValidation>true</useBeanValidation>
							<useOptional>true</useOptional>	
							<hideGenerationTimestamp>true</hideGenerationTimestamp>								
						</configOptions>							
					</configuration>
				</execution>
			</executions>
		</plugin>

`

For the above configuration in openapi-generator\modules\openapi-generator\src\main\resources\JavaSpring\methodBody.mustache an expression of {{#async}}CompletableFuture.completedFuture({{/async}} should resolve to empty. Unfortunately : In openapi-generator\modules\openapi-generator\src\main\java\org\openapitools\codegen\DefaultGenerator.java the value of async in this.config.additionalProperties() is a string and not a boolean.

This causes wrong code generation.

I am providing a immediate fix for this: Please see the code difference in openapi-generator/src/main/java/org/openapitools/codegen/languages/SpringCodegen.java.